Screw-tube conveyors are generally known according to prior art, and defined, for example, in DIN 15 201. In addition to continuous conveyance of bulk material, screw-tube conveyors always also serve for mixing same; in many cases they may also serve for the surface treatment, for surface coating, or for the thermal treatment of the bulk material. Contrary to so-called screw conveyors, which are not an object of the invention, screw-tube conveyors are not very efficient solely for conveying bulk material.
In the traditional manufacture of a screw-tube conveyor a helix is attached to the interior of a cylindrical rotatable tube, for example, welded, soldered, etc., in that persons, or welders crawl into the screw-tube conveyor and carry out the attachment work at the seam between the rotatable tube and the helix.
The length of the attachment zone between the rotatable tube and the helix is many times longer than the total length of the screw-tube conveyor. Traditionally, the attachment zone is formed by a very long weld seam, optionally on two sides, which represents a substantial cost factor in the manufacture of the screw-tube conveyor.
In order to make such attachment even possible, both the exterior diameter and the clear interior diameter of the rotatable tube must have certain minimum values. For this purpose the clear interior diameter of the rotatable tube is determined by the height, or depth of the helixes. The screw pitch may also not be too small so that access to the attachment zone is ensured between the rotatable tube and the helix.